The present invention relates to an image forming apparatus for generating a recording fluid having a predetermined density and/or a predetermined color by changing a mixing ratio of a plurality of inks based on an image signal and leading the thus obtained fluid to an image receiving medium to form an image.
U.S. Pat. No. 4,109,282 (hereinafter referred to as a prior art reference 1) discloses a printer having a structure such that a valve called a flap valve is disposed in a flow channel for leading two types of liquid, i.e., clear ink and black ink onto a substrate for forming an image. The flow channel for each ink is opened/closed by displacing this valve so that the two types of liquid are mixed in a desired density to be transferred onto the substrate. This enables printout of an image having gray scale information which is the same as that of the image information displayed on a TV screen. In this reference is disclosed that a voltage is applied between the flap valve and an electrode disposed on a surface opposite to the flap valve and the valve itself is mechanically deformed by an electrostatic attracting force to cause displacement of the valve. Furthermore, the ink is absorbed in paper by a capillary phenomenon between fibers of the print paper.
U.S. Pat. No. 4,614,953 (hereinafter referred to as a prior art reference 2) discloses a printer head apparatus by which only a desired amount of multiple types of ink having different colors and solvent is led to a third chamber to be mixed therein. In this reference is disclosed that a chamber and a diaphragm-type piezoelectric effect device attached to this chamber are used as means for check-weighing the desired amount of ink and a pressure pulse obtained by driving this piezoelectric device is utilized.
Unexamined Japanese Patent Publication (KOKAI) No. 201024/1993 (hereinafter referred to as a prior art reference 3) discloses an ink jet print head including: a liquid chamber filled with a carrier liquid; ink jet driving means disposed in the liquid chamber; a nozzle communicating with the liquid chamber; and a mixing portion for mixing ink to the carrier liquid in this nozzle. In this reference is also disclosed that adjusting means having a check valve structure for adjusting an amount of mixture of ink to a desired value is provided.
Similarly, Unexamined Japanese Patent Publication (KOKAI) No. 125259/1995 (hereinafter referred to as a prior art reference 4) discloses an ink jet recording head including: first and second supplying means for supplying inks having first and second densities, respectively; and controlling means for controlling an amount of supply of the second ink by the second supplying means so that a desired ink density can be obtained.
In this reference 4, employment of a micro-pump which has an exclusive heating device and is driven by its heat energy is disclosed. As this micro-pump, there is disclosed an example such that the heat energy is generated by the heating device and a pressure obtained by nucleate boiling caused by the heat energy is used to drive, for example, a piston-type valve or a cantilever-like valve. Furthermore, this reference 4 describes that an inflow of ink can effectively be controlled in an area where the inflow is particularly small by adopting an actuator consisting of a shape memory alloy to this valve.
Unexamined Japanese Patent Publication (KOKAI) No. 207664/1991 (hereinafter referred to as a prior art reference 5) discloses a structure which is similar to that in the prior art reference 2 but does not use a third chamber for mixing a plurality of types of ink.
Unexamined Japanese Patent Publication (KOKAI) No. 156131/1997 (hereinafter referred to as a prior art reference 6) discloses an ink jet printer comprising a plurality of printer heads for forming an image having multiple colors based on image data. Ink and diluent are mixed at a predetermined ratio to obtain diluted ink which is jetted from a nozzle so that a recording image is formed on a recording medium. The ink jet printer ejects the diluent from at least one printer head out of the multiple printer heads when all-white image data, that is, data representing that amount of mixture of ink is too small to realize a clear printing density, is input. As a result, a rapid change in tone (a tone jump) is prevented and the additional consumption of the diluent is suppressed to improve drying characteristics.
In the prior art disclosed in the prior art reference 1, the ejection ports for two types of liquid are separately open directly to the print paper, and the respective types of liquid are separately attracted on the print paper by the capillary phenomenon immediately after ejection. Therefore, a quantity of attraction of each liquid on the paper readily fluctuates under influence of a paper quality of the print paper, which results in the unstable image quality or difficulty of formation of an image having high fidelity to the image signal.
In any of the prior art references 2 to 6, a plurality of inks are mixed beforehand or caused to be confluent, and thereafter the mixed liquid (including the confluent liquid) is led onto the print paper. However, when a mixing ratio of one ink is remarkably small (ejection amount is remarkably small), the ink cannot smoothly be confluent with other ink. That is, when the ejection amount of the ink is small, an amount (ingress amount) of the ink advancing to the mixing portion (or a confluence portion) for mixing with the other ink becomes small. Therefore, a leading end of the ink cannot cut in a flow of the other ink by a surface tension of the ink. Therefore, the density of a mixed liquid (confluent liquid) cannot follow the image signal with fidelity, and a problem that an image quality is deteriorated occurs.
Furthermore, in any of the prior arts disclosed in the prior art references 2 to 6, a plurality of inks are brought into contact with one another in the mixing section (the confluence portion), and each ink is ejected by a predetermined amount to be mixed. That is, the ejection port for each ink is formed and assembled in the mixing section. Each ink cannot therefore be prevented from being naturally diffused from one another.
For example, even if a given ink is not ejected into a mixing chamber in accordance with the image signal, this ink is naturally diffused in the mixing chamber. Thus, the density and/or color of the finally mixed ink liquid differs from the image signal, and an image which has fidelity to the image signal cannot be formed. Even if the natural diffusion of the ink is small, distortion of a contact interface occurs due to a vibration in the mixing portion or any other disturbance, and therefore the undesired mixing of ink is facilitated and the above-described problem becomes more prominent.
Additionally, the prior art reference 3 discloses that adjusting means functioning as a check valve is disposed in the vicinity of the opening of the ink channel formed in the mixing portion in order to mainly prevent the inks from being naturally diffused from one another. However, the adjusting means having the check valve structure complicates a print head configuration and leads to problems such as difficulty in manufacturing, reduction of productivity or increase of a manufacturing cost.
Furthermore, the prior art reference 6 discloses that a colorless diluent continues to flow in case of all-white image data in order to avoid a rapid change in tone (tone jump), but the ink which is not colorless and transparent is continuously diffused in this diluent in this case, and hence the above-mentioned problems can not be prevented.
To solve the problem, the present applicant has considered that the small amount of image forming ink is constantly and continuously supplied to the other ink such as a transparent liquid (image non-forming ink) (e.g., Unexamined Japanese Patent Publication (KOKAI) No. 246920/2000 corresponding to EP 101653A2 and U.S. patent application Ser. No. 09/472,970). For example, when the density is controlled in 256 tones, and the image signal indicates xe2x80x9cwhitexe2x80x9d, the image forming ink corresponding to a smallest density is continuously supplied. In this case, the ejection amount of the image forming ink is remarkably small, and the problem occurs that the leading end of the image forming ink does not cut and cannot be mixed (confluent) with the image non-forming ink.
Particularly in this case, when the xe2x80x9cwhitexe2x80x9d image signal continues, the leading end of the image forming ink enters the image non-forming ink by a small amount in accordance with the image signal. When the ingress amount of the image forming ink exceeds a certain limit, the ink is cut and ejected. Therefore, the cut ink soils the image and deteriorates the image quality.
The present invention has been accomplished under the aforementioned circumstances, and an object thereof is to provide an image forming apparatus in which with a small amount of image forming ink, a leading end of the ink is cut well, and an image density having fidelity to an image signal can be obtained, and an image quality can be enhanced.
According to the present invention, the object is attained by an image forming apparatus for ejecting a recording fluid constituted by a plurality of inks from a common ink ejection port while a mixing ratio of the plurality of inks is changed with respect to one pixel based on an image signal, and transporting the recording fluid to an image receiving medium which is moved with respect to the ink ejection port to form an image; said image forming apparatus comprising:
an ink ejection port for ejecting the recording fluid to the image receiving medium;
a first ink channel for supplying an image non-forming ink to said ink ejection port, the image non-forming ink being an ink for forming no image after dried out;
a second ink channel for supplying an image forming ink to said ink ejection port, the image forming ink being an ink for forming the image after dried out; and
a mixing section disposed upstream of said ink ejection port, for mixing the image non-forming ink supplied from said first ink channel and the image forming ink supplied from said second ink channel;
wherein an opening area Ai of said second ink channel is smaller than the opening area of said first ink channel in said mixing section; and the opening area Ai of said second ink channel has the following relationship with a minimum ejection volume Vi of the image forming ink:
Aixe2x89xa61.2xc3x97Vi(⅔).
The image non-forming ink is constantly supplied, and confluent with the image forming ink, until a predetermined density is obtained. Particularly, when a total flow rate of both inks is constant, the ink is steadily transported to the image receiving medium, and this is further suitable for enhancement of the image quality.
Print paper may be used as the image receiving medium, and the image can directly be formed on this print paper. However, it is also possible to provide a drum-like or belt-like intermediate image receiving medium between the ejection port and the image receiving medium. In this case, an ink liquid supplied from the ejection port is loaded onto an intermediate image receiving medium, and then the ink liquid is transferred to the image receiving medium. Preferably, the ink ejection ports may be separately provided for respective pixels aligned in a width direction of the image receiving medium (a direction perpendicular to a moving direction). The ink ejection ports may be formed into a slot-shaped opening which is elongated in the width direction of the image receiving medium when changing the density and/or the color only in the moving direction of the image receiving medium.
The image non-forming ink is or becomes colorless and transparent after dried out and forms no image (hereinafter referred to as image non-forming ink or clear ink), and the density can be controlled by changing a mixing ratio of the image non-forming ink. It is preferable to constantly add the image non-forming ink to the ink liquid so that a supply amount of the image non-forming ink should not become zero. In this case, when decoloration preventing agents such as antioxidant, ultraviolet ray absorber or other components are included in the image non-forming ink beforehand, a color degradation preventing property and other properties can be imparted to the image. A plurality of image forming inks are determined as inks having colors of yellow, magenta and cyan, and the mixing ratio of these inks can be changed during formation of a color image.
When flow rates of a plurality of inks are controlled, an image can be formed having density and/or color varying in both the moving direction and the width direction of the image receiving medium.
A plurality of inks ejected from the ink ejection port may be transported, that is, jetted onto the image receiving medium as droplets by an ink jet mode, but it is also possible to transport/apply the inks to the image receiving medium as a continuous flow instead of the droplets (continuous coating mode). In this continuous coating mode, a flow of liquid can be ejected or extruded as a continuous flow and transported to the image receiving medium through a slot-opening connecting the ink ejection ports provided for the respective pixels in the width direction.
The image forming ink is controlled in such a manner that a volumetric flow rate per unit time does not constantly turn to zero, and it is then possible to smoothly control the small amount of ink. In this case, a minimum addition amount of the image forming ink may be the same as or larger than a flow rate necessary for refreshing a volume of the image forming ink mixed into another ink by natural diffusion. However, the addition amount should be suppressed to such an extent that a change in density and/or color due to addition of this ink does not result in degradation of the image quality. Therefore, it is preferable to set the additional amount in such a manner that a change in optical density of the ink liquid due to addition of this ink is less than 0.1. Here, the optical density means a degree by which a substance absorbs light. When it is assumed that the optical density is represented as D, an intensity of an incident light is I0, and an intensity of a transmitted light is I, the optical density can be defined by D=log10(I0/I). Vibration is preferably absorbed in a portion where a plurality of inks becomes confluent, so that turbulence of a contact interface is prevented from occurring due to vibration and disturbance of the ink, and diffusion may be prevented.
The flow rates of a plurality of inks can be controlled by the various methods. For example, an ink supply pressure with respect to each ink channel can be maintained to be constant while a sectional area of each ink channel can be changed by a piezoelectric device. In this case, a diaphragm valve facing the ink channel is opened/closed by the piezoelectric device. The piezoelectric device can be driven by a mechanical natural frequency (a resonance frequency) of the device itself, and a time period for driving the device is changed by varying a pulse number of this frequency in order to control the flow rate. It is also possible to continuously control a quantity of distortion (opening of the diaphragm valve) of the piezoelectric device by an analog signal and, in this case, the flow rate is controlled by a voltage of the analog signal.
A flow rate supplied to each ink channel may be controlled by changing a discharged quantity of an ink feed pump. For example, the ink feed pump is driven by a pulse motor (stepping motor), and the ink flow rate can be controlled by the driving pulse number of this pulse motor. The ink feed pump includes: at least one check valve disposed in the ink channel; a cavity provided in the vicinity of this check valve; and a movable member for changing a volumetric capacity of the cavity, so that the pump discharges the ink by changing the volumetric capacity of the cavity. Such pump can be used as an ink feed pump.
The check valve used in the ink feed pump may be constituted by a geometrical form by which a resistance relative to an ink flow direction becomes small and that relative a reverse direction becomes large. Such a check valve has no movable portion and can be produced by utilizing a method for manufacturing an integrated circuit or a printed wiring board or that for manufacturing a micro-machine. The ink feed pump may be driven by the pulse motor.
When the ink feed pump driven by the pulse motor is provided, the ink feed pump may preferably be of a volumetric capacity type by which an ejection amount is proportional to a quantity of rotation of the motor. Suitable examples include a pump for squeezing a flexible tube appressed against the inner surface of a circular case on an inner peripheral side by an eccentric ring in a defined direction, a vane pump, a gear pump, and the like.
The ink feed pump disposed in each ink channel can be formed by the piezoelectric device and the check valve. In this case, the piezoelectric device is a diaphragm valve driven by a mechanical resonance frequency inherent to the device. When the pulse number (pulse number in a defined time or a unit time) of the driving frequency of each piezoelectric device is controlled, an ejection volume flow rate from each ink channel can be controlled.
The opening area of the ink channel of the image forming ink in the ink confluence portion is set to be smaller than that of the ink channel of the image non-forming ink. Therefore, an ink ejection length of the image forming ink increases, and the amount of the image forming ink advancing into the another ink increases. Therefore, the leading end of the image forming ink is easily cut, the density having fidelity to the image signal can be obtained, and the image quality is enhanced.
The flow rate (volume flow rate per unit time) of the image forming ink whose ejection amount is minimum is managed so as not to be constantly zero, and a mixture amount of this image forming ink can always be grasped and managed. In this case, since the natural diffusion of the image forming ink with respect to one pixel is considerably short, it is preferable to determine the flow rate required for refreshing the volumetric capacity by the diffusion as a minimum flow rate. As a result, a fluctuation in color and/or density due to natural diffusion of the ink can be suppressed, and a high-quality image can be formed.
In the present invention, the image formed on the image receiving medium includes graphical intelligence patterns such as alphanumeric characters, graphical display, line art, and other image information.
In another aspect of the present invention, there is provided an image forming method for ejecting a recording fluid constituted by a plurality of inks from a common ink ejection port while a mixing ratio of the plurality of inks is changed with respect to one pixel based on an image signal, and transporting the fluid to an image receiving medium which is moved with respect to the ink ejection port to form an image;
wherein said plurality of inks include an image non-forming ink which forms no image after dried out and at least one image forming ink which forms the image after dried out,
an opening area Ai of a channel of said image forming ink is smaller than the opening area of the channel of said image non-forming ink in a confluence of said plurality of inks; and
the opening area Ai of the channel of the image forming ink has the following relationship with a minimum ejection volume Vi of the image forming ink:
Aixe2x89xa61.2xc3x97Vi(⅔). 